1. Field of the Invention
The present invention is directed to an apparatus for setting a rotational torque of a screw mechanism and further relates to a friction mechanism between a lens frame and a lens frame holder.
2. Description of the Related Art
The rotational torque of a screw mechanism mainly depends upon how accurate the screws used in the screw mechanism are made and the length of the engaged portion of the screw threads. When it is necessary to provide a large rotational torque in a screw mechanism, a biasing member or a spring member is generally provided in a space formed between a male screw thread and a female screw thread which engage with each other. However, when a biasing member is provided in a screw mechanism, since providing the above space is necessary, the screw mechanism cannot be kept compact. In the case where a leaf spring is provided as the biasing member in the above space, it is sometimes the case that the desired torque cannot be obtained due to a movement transformation of the leaf spring or the like. For instance, in a lens barrel having such a screw mechanism, the position of a lens (or lenses) in the optical axis direction is adjusted by rotating a screw member which supports the lens (or lenses) during the assembly process of the lens barrel. If the rotational torque of the screw mechanism in the lens barrel is not large enough, the screw thread can be accidentally rotated after the position of a lens (or lenses) in the optical axis direction has been adjusted, and before the screw thread is fixed by means of adhesive or the like so as not to be rotatable, resulting in an inaccurate position of the lens (or lenses).
In general, the photographing lens of a camera, for example includes a movable lens secured to a lens frame, and a lens frame holder which holds the lens frame, so that the lens frame is moved through a roller pin provided on the lens frame, a linear movement guide mechanism and a cam mechanism, etc.
In such a known lens holding mechanism, there is a gap or play in a radial direction between the lens frame and the lens frame holder, thus resulting in a deviation of the lens from the optical axis.